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Claims  |
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What is claimed is:
1. A process for imaging by nuclear magnetic resonance, wherein it
comprises the following stages for giving an image of the molecular
diffusion of an investigated body:
the body is placed in a constant magnetic field B.sub.o ;
the thus positioned body is subject to a first plurality of first spin echo
excitation sequences in the presence of first field gradient sequences,
said first spin echo sequences having an integral number N equal to or
greater than one excitations where the magnetic moments of the nuclei of
the body are flipped by 180.degree. following an excitation in which said
moments have been flipped by 90.degree. to obtain in this way sequences
with N slightly diffusing echoes;
the magnetic resonance signals are recorded at the end of these first
sequences and a first image is calculated corresponding to echo N of these
signals by allocating to each point of the image a value corresponding to
the magnetic resonance signal of the point corresponding thereto in the
body;
the thus positioned body is subject to a second plurality of second spin
echo excitation sequences in the presence of second field gradient
sequences, said second spin echo sequences having at least one 180.degree.
excitation following a 90.degree. excitation for forming diffusing
sequences with at least one echo, the total echo durations of the second
excitation sequences being equal to the total echo durations of the first
excitation sequences;
the magnetic resonance signals are recorded at the end of said second
excitation sequences and a second image is calculated corresponding to the
echoes of these signals by allocating to each image point a value
corresponding to the magnetic resonance signal of the point corresponding
thereto in the body;
there is a point-by-point comparison of the values allocated for the first
image with the values allocated for the second image, in order to produce
a third image representing the molecular diffusion at each point of the
body.
2. A process according to claim 1, wherein the second sequences have longer
and/or more powerful field gradient pulses out of the presence of spin
echo excitations and oriented according to one axis in order to form field
gradient supplements.
3. A process according to claim 2, in which the field gradient sequences
incorporate field gradient pulses oriented along three axes X, Y, Z, the
axis Z of the constant B.sub.o or selection axis, as well as two axes X, Y
orthogonal to said axis and respectively called phase coding axis Y and
reading axis X, in which the method for calculating the images is of the 2
DFT type, wherein the axis of the supplements is the reading axis.
4. A process according to claim 2, in which the field gradient sequences
incorporate field gradient pulses oriented along three axes X, Y, Z, the
axis Z of the constant field B.sub.o or selection axis, as well as two
axes X, Y, orthogonal to said axis and respectively called phase coding
axis Y and reading axis X, in which the method for calculating the images
is of the 2 DFT type, wherein the axis of the supplements is the selection
axis.
5. A process according to claim 2, in which the field gradient sequences
incorporate field gradient pulses oriented in accordance with three axes
X, Y, Z, the axis Z of the constant field B.sub.o or selection axes, as
wall as two axes X, Y, orthogonal to said axis and respectively called the
phase coding axis Y and the reading axis X, in which the method for
calculating the images is of type 2 DFT, wherein the axis of the
supplements is the phase coding axis.
6. A process according to the claim 1, wherein it is performed on a number
of occasions for forming third multisection images of the studied body.
7. A process according to the claim 2, wherein the application times of the
gradient supplements are, within a same sequence, as timely spaced as
possible before and respectively after the 180.degree. radio frequency
excitation time.
8. A process according to the claim 1, wherein a standard body is placed
alongside the body to calibrate the calculations of the third image.
9. A process according to the claim 1, wherein the durations of the echo
time T.sub.E, of the first spin echo sequences are all equal to one
another.
10. A process according to the claim 1, wherein N is equal to four.
11. A process according to claim 2, wherein the stages relative to the
second image are modified with gradient supplements oriented along another
axis in order to produce another third image for determining the nature of
the imaged regions.
12. A process according to the claim 11, wherein the logarithm of the ratio
of the values is calculated for comparison purposes.
13. A process according to claim 1, wherein the thus positioned body is
subject to a third plurality of third spin echo excitation sequences in
the presence of third field gradient sequences, the third spin echo
sequences having at least one 180.degree. excitation following the
90.degree. excitation for producing diffusing sequences with at least one
echo, the total echo durations of the third excitation sequences being
equal to the total echo durations of the first excitation sequences, the
third field gradient sequences differing from the second field gradient
sequences; the magnetic resonance signals are read at the end of said
third excitation sequences and a fourth image is calculated corresponding
to the echoes of these signals by attributing to each image point a value
corresponding to the magnetic resonance signal of the point corresponding
thereto in the body;
the values attributed for the first image are compared point-by-point with
the values attributed for the fourth image for producing a fifth image
representing the molecular diffusion at each point in the body;
and then the values attributed for the third image are compared
point-by-point with the values attributed for the fifth image for
producing a sixth image representing the true molecular diffusion in the
body and which is free from micro-circulation interference.
14. A process according to claim 1, wherein the thus positioned body is
subject to a third plurality of third spin echo excitation sequences in
the presence of third field gradient sequences, the third spin echo
sequences having at least one 180.degree. excitation following the
90.degree. excitation for producing diffusing sequences with at least one
echo, the total echo durations of the third excitation sequences being
equal to the total echo durations of the first excitation sequences, the
third field gradient sequences differing from the second field gradient
sequences; the magnetic resonance signals are read at the end of these
third excitation sequences and a fourth image is calculated which
corresponds to the echoes of these signals by attributing to each image
point a value corresponding to the magnetic resonance signal of the point
corresponding thereto in the body;
the values attributed for the first image are compared point-by-point with
the values attributed for the fourth image for producing a fifth image
representing the molecular diffusion at each point of the body;
and the values attributed for the third image are compared point-by-point
with the values attributed for the fifth image in order to produce a
seventh image representing a perfusion phenomenon in the body.
15. A process according to claim 13 further comprising a step wherein the
values attributed for the third image are compared point-by-point with the
values attributed for the fifth image in order to produce a seventh image
representing a perfusion phenomena in the body and wherein the sixth and
seventh images are simultaneously produced.
16. A process according to any one of the claims 1 to 14, wherein the
diffusing sequences are sequences with a single spin echo.
17. A process according to any one of the claims 11, 13 or 14 wherein the
effect of the speed of moving parts of the body created by so-called
interfering field gradient sequences is modulated by applying, before the
record of the signal, a compensating magnetic field sequence, whose
integral calculated on its duration is zero and whose history and value
are a function of the history and value of the interfering fields.
18. A process according to claim 17, wherein the sequence of the
interfering magnetic field incorporates magnetic field pulses along
orthogonal axes X, Y, Z, and wherein the sequence of the compensating
magnetic field incorporates magnetic field pulses along these three same
axes, in order to modulate one by one the effects of the speed of the
moving parts of the body along these three same axes.
19. A process according to claim 18, wherein the compensating magnetic
field pulses are determined a priori in a form, in a duration, and in a
position and wherein their amplitude .lambda. is evaluated to obtain the
sought modulation.
20. A process according to claim 17, wherein the sequence of the
compensating magnetic fields incorporates bipolar pulses pairs.
21. A process according to the claim 17, wherein the sequence of the
compensating magnetic fields has pulse pairs, each pulse of a pair having
a value, shape, duration and signal equal to the other pulse of the pair,
said pulses being respectively timely located before and after a second
high frequency pulse.
22. A process according to the claim 17, wherein the integral of the
product of the value of the interfering field pulses by the time
separating them from the recording of the emitted signal as compensated by
the integral of the same product obtained with compensating field pulses
in lieu of the interfering field pulses.
23. A process according to claim 20, wherein the pulses of the pulse pairs
comprise pulses which are as timely spaced as possible from one another.
24. A process according to the claim 17, wherein the effect of the speed of
the moving parts of the body produced solely in the second field gradient
sequences is modulated. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an imaging or image formation process by
nuclear magnetic resonance. The use of this process is more particularly
intended in the medical field for representing sections of organs of the
human body.
2. Discussion of Background
Nuclear magnetic resonance imaging has mainly been developed as a means of
medical diagnosis. It makes it possible to display internal tissue
structures with a contrast and resolution of a quality not hitherto
achieved with other imaging processes. In order to obtain an image by
nuclear magnetic resonance of an organ section with differentiation of the
tissue characteristics of the organ, use is made of the property of
certain atomic nuclei, such as protons, of orienting their magnetic moment
whilst acquiring energy when placed in a main constant magnetic field
B.sub.0. A particular zone of an object containing nuclei then has an
overall magnetic moment, which can be flipped in accordance with a given
orientation, perpendicular or parallel to field B.sub.o, by inducing a
resonance by the emission of a radio frequency field perpendicular to the
main field.
All the particles which then have a magnetic moment rotating at a so-called
Larmor precession speed tend to find again the initial orientation
parallel to B.sub.o by emitting a radio frequency signal at the
characteristic resonant frequency of B.sub.o and of the nucleus. This
signal can be detected by a receiving antenna. The duration of the return
to equilibrium of the overall magnetic moment of a considered region and
the decrease of the signal are dependent on two important factors, namely
the spin-system interaction and the spin-spin interaction of the particles
with the surrounding material. These two factors lead to the definition of
two relaxation times, called respectively T.sub.1 and T.sub.2. A
considered region of an object thus emits a signal, whose intensity is
dependent on T.sub.1, T.sub.2, the proton density of the region and the
time which has elapsed since radio frequency excitation.
In order to locate a region of the organ, it is necessary to establish the
nature of its emission as a function of the local conditions of the
magnetic field. These local conditions are imposed in such a way that the
frequency and the phase of the emission are characteristic of the location
in space of said region of the organ. For this purpose, pulsed magnetic
field gradients are superimposed on the main field B.sub.o. These
gradients are oriented in directions X, Y and Z in order to define, at all
times, the volume elements which resonate at known frequencies. For
obtaining a complete picture, the local conditions are imposed in
programmed sequences, which are stored in a master computer. These
sequences define the application times of the gradients, the excitation
times of the nuclei by the radio frequency field pulses and the reading or
acquisition times of the image data.
Another factor intervenes to modify the intensity of intercepted signal
when the nuclei return to their equilibrium orientation. This other factor
depends upon the molecular diffusion or scattering of the medium. The
molecular diffusion relates to the displacements undergone by the
molecules of a medium as a function of time. The inhomogeneity of the
magnetic field in which these molecules are located then has the effect
that the magnetic resonance frequency of these molecules changes. Thus,
this frequency is linked with the gyromagnetic ratio of these molecules at
the intensity of said field. Furthermore, during a magnetic resonance
experiment, particularly one with a sequence of spin echoes, the
intercepted signal is below the expected intensity.
Thus, the frequency of occurrence of the molecules of the region of the
space where the magnetic fields differ has the effect of modifying the
relative phases of the contributions made by each of these molecules to
the intercepted overall magnetic resonance signal. As the displacements of
the molecules are in all directions, the phase dispersion resulting
therefrom has the effect that certain contributions are mutually opposed.
The intercepted signal is then weaker. This sensitivity loss to a certain
extent represents the diffusion characteristic of a medium and a highly
diffusing medium is subject to a very rapid decrease in its magnetic
resonance signal with the echo time used.
In human organs there are pathological tissues, e.g. angiomas and tumors
having often identical standard nuclear magnetic resonance signals. In
other words, the images of these organs show the relaxation times T.sub.1
or T.sub.2 and do not make it possible to discriminate these
conformations. Thus, the examination of the standard image does not make
it possible to make a therapeutic decision. Thus, the aim of the present
invention is to propose images where the parameter shown is the molecular
diffusion characteristic in the studied tissues, in order to improve their
differentation.
DESCRIPTION OF THE PRIOR ART
The book "Biomedical Magnetic Resonance", published by Radiology Research
and Education Foundation, San Francisco 1984 contains an article by George
Wesbey et al and entitled "Translational Molecular self-diffusion in
Magnetic Resonance Imaging: Effects and Applications". This article
suggests measuring the diffusion constant of the regions of a medium by
comparing the relative effect of the diffusion on the studied medium and
on a standard substance during different magnetic excitation sequences. In
the description given of this method, a disadvantage appears. Thus, these
sequences are obtained by increasing the intensity of a sections selection
gradient, which modifies the thickness of the studied section. This method
then only applies to objects which are finer then the finest section
thickness obtained by the sequences used, so that it is not usable in man.
Moreover, the sensitivity of this method to diffusion is relatively
limited (short echo times, ineffectively placed gradients in the
sequence), so that the authors use several acquisitions for obtaining a
reasonable accuracy regarding the measurement. Further, it is necessary to
use the same standard to act as a reference for the measurements.
SUMMARY OF THE INVENTION
The present invention makes it possible to establish a diffusion image
whilst avoiding these disadvantages. In particular, the images have a
constant thickness section, which makes it possible to carry out
acquisitions on man, even with a multisection process. The sensitivity to
diffusion is good and is due to the use of a relatively long echo time and
effective gradients as a result of their intensity and position. Moreover,
the exact determination of diffusion coefficients is obtained without a
standard substance. In the invention, the absolute effect of the diffusion
has been calculated from acquisition parameters.
The present invention therefore relates to a process for imaging by nuclear
magnetic resonance, wherein it comprises the following stages for giving
an image of the molecular diffusion of an investigated body:
the body is placed in a constant magnetic field B.sub.o ;
the thus positioned body is subject to a first plurality of first spin echo
excitation sequences in the presence of first field gradient sequences,
said first spin echo sequences having an integral number N equal to or
greater than one of excitations where the magnetic moments of the nuclei
of the body are flipped by 180.degree. following an excitation in which
said moments have been flipped by 90.degree. to obtain in this way
sequences with N slightly diffusing echoes;
the magnetic resonance signals are recorded at the end of these first
sequences and a first image is calculated corresponding to echo N of these
signals by allocating to each point of the image a value corresponding to
the magnetic resonance signal of the point corresponding thereto in the
body;
the thus positioned body is subject to a second plurality of second spin
echo excitation sequences in the presence of second field gradient
sequences, said second spin echo sequences having a 180.degree. excitation
following a 90.degree. excitation for forming diffusing echo sequences,
the echo duration of the second excitation sequences being equal to the
total echo duration of the first excitation sequences;
the magnetic resonance signals are recorded at the end of said second
excitation sequences and a second image is calculated corresponding to the
echoes of these signals by allocating to each image point a value
corresponding to the magnetic resonance signal of the point corresponding
thereto in the body;
there is a point-by-point comparison of the values allocated for the first
image with the values allocated for the second image, in order to produce
a third image representing the molecular diffusion at each point of the
body.
BRIEF SUMMARY OF THE DRAWINGS
The invention is described in greater detail hereinafter relative to
non-limitative embodiments and the attach drawings, in which the same
references designate the same elements throughout and wherein show:
FIG. 1. a device suitable for performing the imaging process according to
the invention.
FIG. 2a. time diagrams of the field gradient and excitation sequences
applied to the body for producing the first and second images.
FIGS. 2b and 2c. time diagrams of so-called compensated homologous
gradients, used for increasing the sensitivity to the diffusion effect.
FIGS. 3a and 3b. the paths of the signals recorded respectively at the end
of the first and second excitation sequences.
FIG. 4. a machine for realizing in the invention a modulation process
according to an improvement.
FIG. 5. time diagrams of radio frequency excitation signal, interfering
magnetic field signals and signals recorded in a special measurement
involving type 2 DFT imaging of a section of a body being examined.
FIGS. 6a and 6b. time diagrams of the resulting phase displacements,
following application of interfering magnetic field sequences between the
contributions emitted by fixed particles and moving particles.
FIG. 7. a diagrammatic representation of the response part of a medium,
whose particles are displaced as a function of whether said displacement
is parallel or perpendicular to an imaged sectional plane.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a device for performing the imaging process according to the
invention. This device incorporates means symbolized by a coil 1 for
subjecting a body 2 to a high constant magnetic field B.sub.o. This device
also has generating means 3 and coils 4 for subjecting the thus positioned
body to spin echo sequences with one or more echoes in the presence of
field gradient sequences (FIG. 2a). Coils 4 represent radio frequency
coils and field gradient coils. It also has reception means 5 connected to
the coils 4 for receiving the magnetic resonance signal and means 6 for
calculating and storing a first image I.sub.1 and a second image I.sub.2
relative to two series of experiments imposed by controls C.sub.1 and
C.sub.2 of the generating means 3. In comparison means 7, a point-by-point
comparison is made of images I.sub.1 and I.sub.2 by calculating the
logarithm of the ratio of values representing the magnetic resonance
signals. The means 7 then produce a third image I.sub.3, in which two
regions 8 and 9 of the medium at the location of the imaged section have
different diffusion value responses, whereas they could have had identical
responses in standard magnetic resonance image. These images can be
displayed on a visual display 50.
The process for calculating images I.sub.1 or I.sub.2 is of a conventional
nature. In an example, the imaging method used by means 6 is a so-called 2
DFT method. This imaging method makes it possible to obtain at present the
best image quality. In this method only one sectional plane is excited at
the same time by the radio frequency excitation means (90.degree. or
180.degree.) of a particular form and in the presence of a so-called
selection gradient. In FIG. 1, the selection gradient can be oriented
along axis Z to select a cross-section i.e. in accordance with a plane X,
Y. The principle of 2 DFT imaging is the phase coding of the different
signals acquired. This is obtained by a pulse having a variable intensity
with a so-called phase shift gradient, whose axis is perpendicular to a
reading gradient, whose direction is constant. For example, for a
cross-section, the reading gradient could be gradient X and the phase
shift gradient Y. Then by a double spatial Fourier transform, the image is
constructed hence the name of the method. A description is given of this
imaging procedure in the book "Imagerie par resonance magnetique", M. LE
BIHAN, published by Editions MASSON, Paris, March 1985. An improvement to
this method can make it possible to simultaneously obtain the images of
several parallel sections.
FIG. 2 shows field gradient sequences along the three axis Z, Y and X, as
well as the times of applying radio frequency excitations tending to flip
the spins of the nuclei by 90.degree. for small excitations and
180.degree. for large excitations. For the purpose of imaging a section of
body, spin echoe excitation sequences have to be performed in the presence
of field gradient sequences and the number thereof must be large enough to
ensure that the resolution of the expected image is more precise. At each
excitation sequence, the phase shift gradient Y varies by successive steps
starting from a certain value and extending up to the same value, but with
a different sign. This value is dependent on the shape and duration of
reading gradient 10. This phase shift gradient makes it possible to rotate
each spin by a variable phase, dependent on its ordinate along axis Y and
the value of said gradient. For each image I.sub.1 and I.sub.2, gradient Y
can successively assume the same number of values and in a preferred
manner the definition of the two images is the same.
What differs in the invention between the first image I.sub.1 and the
second image I.sub.2 is the number of spin echo radio frequency excitation
sequences and/or the intensity and shape of the gradient sequences. The
calculation performed by comparison means 7 is linked with the way in
which the images have been acquired and with the information content of
said images. In the first spin echo excitation sequences, N excitations 12
of 180.degree. succeed a single excitation 11 of 90.degree.. The number N
is equal to or greater than 1. In the second spin echo excitation
sequences, there is only a single 180.degree. pulse, which follows the
90.degree. excitation. Moreover, the duration of the echo time T.sub.E of
the second sequence of spin echo excitations is equal to the N durations
of echo time T.sub.E, of the first sequence of spin echo excitations. For
reasons of simplicity in the first spin echo sequences, the successive
echo times are equal to one another. It is easier to divide a given
duration T.sub.E into an integral number of equal elementary durations
T.sub.E '. However, if the successive echo times T.sub.E, of the fist spin
echo sequences are not equal to one another, it is still possible for the
invention to function; it being important for the duration between the
90.degree. pulse 11 and the measuring time 10 to be the same in both
cases.
In the example shown in FIG. 3a, the first spin echo sequence has four
equal echo times T.sub.E', in the centre of which there is a 180.degree.
pulse on each occasion. In FIG. 3b, during a same duration T.sub.E a
single 180.degree. pulse 26 is interposed. In an example, time T.sub.E is
equal to 112 milliseconds and time T.sub.E, 28 milliseconds. In this
example, the repetition time TR, which is the time separating for either
image each spin echo sequences during which gradient Y assumes a different
value, is equal to 1 second. The value of 112 milliseconds is made
sufficiently large to obtain a good sensitivity on the diffusion effect
and a reasonable signal-to-noise ratio. Such excitation-measurement
sequences can be brought about by controls C.sub.1 and C.sub.2, which are
dependent on the machines used.
In the invention, it has been found that the contribution of part of the
medium to the magnetic resonance signal recorded at the end of the echo
time N.T.sub.E' in a first excitation sequence is in the following form:
##EQU1##
In these expressions .rho. is the proton density at the considered
location, .gamma. is gyro magnetic ratio of the molecules of the medium at
said location and G.sub.ij and d.sub.ij correspond to a pair of
compensated homologous gradients, like those shown in FIG. 2b or 2C. A
pair of compensated homologous gradients corresponds to a pair of
gradients of intensity G.sub.ij and G'.sub.ij and durations d.sub.ij and
d'.sub.ij, so that G.sub.ij.d.sub.ij =G'.sub.ij.d'.sub.ij. If the
gradients are placed on either side of a 180.degree. radio frequency
excitation pulse, intensities G.sub.ij and G'.sub.ij are the same sign.
However, if the gradients are placed on the same side, they are of
opposite sign. The sums relate to the number m of compensated homologous
pairs of gradient pulses and the number N of spin echoes. For example, in
FIG. 3b, there are four compensated homologous pairs of gradient pulses to
which, for calculating m, it is necessary to add the compensated
homologous gradient pairs of corresponding to a normal imaging sequence.
For the second excitation sequence N.T.sub.E' is replaced by T.sub.E and
N is replaced by 1. Parameters G.sub.ij, d.sub.ij and m are then different
therein. In this case, the signal is S.sub.1 (T.sub.E). The contributions
of a point of the medium are supplied in the magnetic resonance signal in
common with the contributions of the other points of the examined section
in the medium. The 2 DFT imaging method makes it possible to definitively
allocate to each point of the images I.sub.1 or I.sub.2 a value
representative of these contributions.
FIG. 3a represents by the envelope curve 13 the theoretical signal
resulting from the existence of a relaxation time T.sub.2 when all the
other parasitic effects, including the diffusion, can be ignored. Curve 14
shows what happens to this theoretical curve when it is measured with
so-called slightly diffusing sequences, i.e. when the diffusion effect is
not very noticeable. This effect can be noticed if number N is
sufficiently high for a given time T.sub.E (=NT.sub.E'), e.g. it is
equivalent to 4 here. Thus, in the contribution formula, the higher N, the
smaller the coefficients d.sub.ij and I.sub.ij and the weaker the
diffusion effect. The radio frequency excitations designated 15 to 19 in
FIG. 3a involve an excitation tending to make the nuclei of the flip by
90.degree. (excitation 15) and four successive excitations tending to flip
these nuclei by 180.degree. (16 to 19). The dotted line signal 20 is the
nuclear magnetic resonance signal following the first 90.degree.
excitation. The peaks 21 to 23 of this signal, measured after successive
echo time periods T.sub.E, follow curve 14. Signal S.sub.N of the
preceding formula corresponds to the amplitude of peak 24.
FIG. 3a also shows the envelope 25 of these peaks when the body is subject
to diffusing sequences. Curve 25 is also plotted in FIG. 3b. In a
diffusing sequence, a single 180.degree. excitation 26 follows the
90.degree. pulse 15 and the signal measured at the end of echo time
T.sub.E is S.sub.1. The value of this signal is given by a formula
identical to the preceding formula, the only difference resulting from the
fact that in this case N=1. On examining these two drawings, it is
apparent that it is possible by comparing an image I.sub.1, relative to
peaks following envelope 14, with an image I.sub.2, relative to peaks
following a curve 24, to represent an image I.sub.3 relative to the
difference or variation 27 between these two curves. To the extent that
the slightly diffusing sequence can be increasingly less diffusing, curve
14 moves towards curve 13. This makes it possible to represent an image,
in which the diffusion effect 28 only can be better revealed. It is useful
to note that the third image with the process according to the invention
differs from the teaching given in the aforementioned document. With that
teaching, the signal is proportional to a variation 29 between the effects
of the diffusion in the studied body and a standard body, whose diffusion
response is entered in the broken line curve 30.
According to the invention, the ratio of signals S.sub.N and S.sub.1 is
formed and the logarithm of said ratio is taken. It is thus possible to
produce a representative signal (point-by point in the image)
corresponding to the following formula:
##EQU2##
where D is the molecular diffusion constant.
This formula shows that the thus performed processing on S.sub.N and
S.sub.1 gives, to within a factor K, the measurement of the diffusion
constant at the considered location in the image. However, this ratio only
gives the above result to the extent that, on the one hand the sampling
times of the signals T.sub.E and N.T.sub.E' are equal to one another and
on the other hand where the repetition time TR is the same for the
sequences with N echoes and for those with one echo. The function f then
takes on the same value for the two sequences and it disappears in the
calculation of image I.sub.3, where only the diffusion effect appears.
In order that the diffusion effect can be noticed in curve 25, it is of
interest to have an echo time T.sub.E in both sequences which are
sufficiently long and/or to use greater and longer gradients than in the
standard sequence. It is in particular possible to use supplementary
gradients 41 to 48 with a maximum spacing from the 180.degree. radio
frequency excitation pulses outside the periods during which the radio
frequency excitations are applied or received. However, in order that the
diffusion effect in the non-diffusing sequence minimized, it can assume a
sufficiently large number N and N=4 would appear to be adequate.
A supplementary gradient is shown in FIG. 2a below the radio frequency
excitation graph. It can be applied to any one of the axes X, Y or Z. For
example, it can be applied to the selection or phase coding axis. When
applied to the selecton axis, it does not interfere in the thickness of
the selected section in the body to the extent that its supplementary
application takes place outside the application times of the radio
frequency excitations during which the selection gradient is present.
Thus, if the selection gradient receives a value supplement during the
90.degree. selection excitations and the 180.degree. return, it will lead
to a greater inhomogeneity. For the same given pass band of the
excitation, the thickness of the excited section in body 2 will be finer
and the signal restored by this finer section will be weaker. If it is
wished that it should be just as strong, it is necessary to increase the
excitation pass band and this process then cancels out the simplicity of
the method. In a preferred manner, the supplementary gradient is applied
as a supplement of the reading gradient. In a preferred manner, everything
else being equal, it will be ensured that the gradients will be added at
periods which are as remote as possible from those when flip excitations,
i.e. excitations 12 are applied.
FIG. 3b gives an idea of the times during which it is possible to apply
field gradient supplements 41 to 48 in a preferred manner. They are
applied outside the times during which the peaks of the nuclear magnetic
resonance signal are recorded. In the representation, gradients 41 and 48
are asymmetrical. However, as they are compensated homologs compared with
the flip pulse 26, their integrals with respect to time are equal. The
large pulses 41 and 48 can be used alone and are as remote as possible
from excitation 26.
The choice of the axis on which the supplementary pulses of the gradients
are applied may be indifferent. If it is considered that the molecular
diffusion is an isotropic phenomenon, this is effectively the case.
However, if it is considered that the molecular diffusion can in certain
cases be anisotropic, it can be of interest to choose the axis on which
the diffusion effect is to be aided. In particular, certain human body
tissues have a preferred orientation as a result of their location in the
body. This preferred orientation results from a preferred form of the
cells forming them. These cells which have no reason for being symmetrical
on three axes then have different molecular diffusion coefficients in each
of the three axes. By comparing the molecular diffusion images obtained
according to the process of the invention and the field gradient
supplements applied along one axis and then along another axis, it can be
subsequently possible to determine what type of tissue is involved.
The measurement of the given diffusion in the third image is a quantitative
measurement on the basis of the calculation of the coefficients b.sub.km
(k being equal to 1 or N according to the sequences) and consequently of
the factor K. However, the imperfections inherent in the imaging systems
can lead to an overall attenuation of the signals S.sub.n or S.sub.1 at
each point of the image, e.g. slightly different section thickness, poor
rephasing of the section selection of one of the two images, etc. This
overall attenuation can be calibrated with the aid of a standard substance
33 position along the studied body, so that it appears on the edges of the
image field. If Se.sub.N and Se.sub.1 are magnetic resonance signals in
the standard substance, corresponding to signals S.sub.N and S.sub.1 in
the studied substance, respectively at the end of slightly diffusing and
diffusing sequences, it is possible to write that the molecular diffusion
coefficient of a point in the section is equal to:
##EQU3##
In which De is the known molecular diffusion coefficient of the standard
substance, under these conditions, calibration is obtained in a simple
manner.
Thus, in a living tissue two displacement phenomena occur. A first
phenomenon is due to the molecular diffusion, as pointed out up to now. A
second phenomenon is due to the micro-circulations in the tissues and
mainly result from vascularization. These micro-circulations disturb the
molecular diffusion image. Furthermore, in a variant the invention is
performed by modulating the effect of the speed of the moving parts (the
blood) of the tissue according to a modulation process described
hereinafter, which makes it possible to modulate the effect (on the
resonance signal) of these microcirculations. In this modulation process
it is known that, by adding so-called compensating bipolar gradients, the
image of the molecules of the blood is taken into account as if they where
fixed. Under these conditions the diffusion phenomenon appears alone. This
modulation process has previously been described in French patent
application 85-12352, filed on Aug. 13, 1985. The content of this
earlier-dated application now forms an integral part of the present
invention.
For the thus made improvement, it is possible to explain that the
micro-circulations (with slow movements) disturb the diffusing sequences
due to the sufficiently marked sensitization gradients of said sequences.
Thus, in a preferred manner, it is the diffusing sequences which are
compensated. The question arises as to how it is possible to know that
this compensation, sought because it neutralizes the undesirable effect of
the micro-circulations, does not at the same time neutralize the diffusion
effect. The inventors of the present invention think that this
non-neutralization is due to an incoherent time distribution of the
molecular diffusion displacements. However, the micro-circulations are
coherent displacements, because their speeds are quasi-constant. In other
words, the reduction of the resonance signal measured during a diffusing
sequence (compared with that measured during a non-diffusing sequence) is
now due solely to the diffusion and not also to the micro-circulations,
whereof the effect has been neutralized.
The improvement relates to a process for modulating the effect of the speed
of moving parts of a body in a density measurement by nuclear magnetic
resonance (NMR), as well as to the performance of the process for deducing
therefrom the speed of the moving parts in question. The improvement is
more particularly used in the medical field, where the bodies examined are
human bodies and where the moving parts are cells of the blood circulating
in the veins and arteries, or moving organs such as the cardiac muscle. In
this application, the improvement can be more particularly realized with
an imaging or image production process in order to give an image
representing the distribution of the speeds of the moving parts in a
section of the body examined.
During a resonance experiment, if the orienting field B.sub.O is perfectly
homogeneous, in response, mobile particles in a considered region emit a
signal identical to that of the fixed particles of said region. However,
if the orienting field is not homogeneous, or, more generally if for
various reasons (particularly for carrying out image formation) during or
after radio frequency magnetic excitation, an interfering magnetic field
is applied which has an intensity gradient, it is possible to show that
the contributions made by the mobile particles in the overall signal
emitted are affected by a phase component dependent on the speed thereof.
This can be easily understood. The resonant signal emitted vibrates at a
frequency f.sub.O, which is dependent on the intensity of the orienting
magnetic field B.sub.O and the gyromagnetic ratio characteristic of the
medium in question .gamma.. All variations in the intensity of the field
B.sub.O consequently lead to a corresponding variation of the resonant
frequency. Consequently a fixed particle which, following radio frequency
excitation, is exposed firstly to the field B.sub.O resonates at a
frequency f.sub.O and then secondly is exposed to a stronger field B.sub.O
+.DELTA.B.sub.O, resonates at a higher frequency f.sub.O +.DELTA. f.sub.O.
Thirdly it is again exposed to field B.sub.O and it again vibrates at
frequency f.sub.O. During the latter the signal emitted is then phase
displaced with respect to its phase initially. This phase displacement is
proportional to the amplitude of the interference .DELTA.B.sub.O and to
the duration of said interference. If all the particles of the medium are
fixed on or if the interference which has reached all the medium does not
have a gradient, this simply means that the overall signal emitted is
delayed.
However, the procedure is quite different in the case of particles having a
certain speed when the interference has a gradient. During three periods
and as a result of the displacement speed thereof during these periods,
they occupy regions in space where the orienting and interfering fields
differ. They differ respectively as a result of the existence of
inhomogeneities or the fact that gradients exist. Therefore the
contribution of the mobile particles in the signal is provided with a
phase dependent not only on the amplitude of the interference encountered
(as for fixed particles), but also the amplitude variation of said
interferences along the path which they have taken. This variation, which
constitutes the gradient is geographically imposed. Consequently the phase
displacement of the signal of the mobile particles is then dependent on
their speed, because the higher their speed the more regions in space they
occupy. If the displacement speeds, inhomogeneity or field gradients are
too large, the phases of the different contributions can be affected to
this point that they end up by providing opposition. In this case, these
contributions are mutually cancelled out and the resulting overall signal
is not as strong. In practice this effect is such that it often gives the
illusion that there is no matter in a body at the location where the
mobile particles circulate.
To reveal the existence of mobile particles and to measure their
characteristics, the density and possibly the displacement speed, it is
possible to proceed in accordance with a method described by E. L. HAHN in
February 1960 in the Journal of GEOPHYSICAL RESEARCH, vol. 65, no. 2, p.
776 ff. The author suggests subjecting the medium in question to a
sequence of a particular gradient and coding it. The principle of this
coding consists of applying following the flipping of the radio frequency
pulse, a bipolar gradient along the axis of a velocity component which it
is wished to recognize. A bipolar gradient is such that its time integral
is zero from the time corresponding to the start of the radio frequency
pulse to the time corresponding to the measurement. The magnetic moment of
the spin of a stationary particle in this case only undergoes a zero
overall phase displacement. Thus, the phase displacement undergone during
the application of the first part of the bipolar gradient is compensated
by the application of the second part of said gradient. However, a mobile
particle with a positive speed along the gradient axis then undergoes
during the second part of the pulse, a larger phase displacement in
absolute values than during the first part. The reason is that during this
second part, it frequents a region in space where, due to the gradient,
the interfering magnetic field is stronger. By comparing a measurement
made with such a bipolar gradient and a measurement made without it being
applied, it is possible to deduce therefrom the speed and number of mobile
particles.
Whatever the objectives pursued, simple measurement or measurement with an
image and no matter what the procedures adopted, the sensitivity of the
speed phenomenon to the interfering magnetic field applied is such that
the displacement phenomena can only be revealed when the maximum speeds
are below a limit. Particularly in image formation, depending on whether
the velocity component to be revealed is parallel or perpendicular to the
plane of the imaged section, the sensitivity of NMR machines is at present
approximately 1 radian (cm/s) to 0.2 radian (cm/s). This means that a
particle moving at 1 cm/second in the plane of the section contributes to
the overall signal emitted with a phase displacement of 1 radian compared
with the contributions emitted by the fixed particles. In the human body a
nominal blood circulation speed of 50 cm/s is reached at present, whereby
it can even be several meters per second in the heart. Moreover, the
distribution of the speeds in a vessel ranges between zero on the edges of
the vessel and nominal speed at the centre of the vessel. Thus, each
particle of a vessel contributes to the signal with a phase displacement
which can be zero to 50 radians. Knowing that contributions phase
displaced by .pi. radians mutually oppose one another, the resulting
signal is zero, which amounts to taking the mean value of a sinusoidal
signal over several periods or cycles. For example, Paul R. Moran in an
article in Radiology of RSNA, 1985, 154, pp. 433-441 refers to a
measurement of a mean spe | | |
